1. Field of the Invention
The present invention relates to a method and apparatus for preparing samples for X-ray spectrometry analysis including and simultaneously loss on ignition/gain on ignition analysis, and more particularly, to a method and apparatus for preparing multiple samples for X-ray spectrometry analysis in a thermogravimetric analyzer of the type having a carousel for supporting and sequentially moving the sample holders within the furnace for weighing, in which the apparatus automatically mixes the material to be analyzed with flux to form the sample in each sample holder by repeatedly tilting the sample holders in different directions, as the carousel is moved, and in some instances, agitating the contents of the sample holders by rapidly moving the carousel back and forth with sudden stops.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
X-ray fluorescence (XRF) is the emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science and archaeology.
The fusion bead method is an effective sample preparation technique for accurate XRF analysis results of ores, rocks and refractory materials since the technique eliminates heterogeneity due to grain size and mineralogical effect. The sample to be analyzed is in powder form and generally is dissolved with a lithium borate flux in a ratio 1:5, 1:10, or 1:14 at approximately 1050 C. The ratio is very important and weight precision of flux and sample within the 1-0.1 mg range is generally required.
Samples are usually never pure and loss on ignition/gain on ignition analysis is often performed with the addition of water of crystallization. Mathematical models are available for the XRF analyzer to correct deviations in the sample to flux ratio of the material as a result of sample weight changes due to volatilization.
Nevertheless, it is quite common to perform loss on ignition/gain on ignition analysis prior to XRF analysis and to introduce the results of the loss on ignition/gain on ignition analysis to the XRF spectrometer to obtain accurate results.
Preparation of fused samples (commonly referred to as “beads”) is a tedious and time consuming task. It requires precision sample weighing, precision flux weighing, dangerous manipulation of very hot components, and cleaning of crucibles in citric acid.
If loss on ignition/gain on ignition analysis is required, an additional operation is needed to analyze the loss on ignition/gain on ignition valve of a different part of the same sample and provide this information to the XRF spectrometer for proper analytical results.
Thermogravimetric analyzers (TGA) are well known in the art as being used to analyze the moisture and other volatile content of ash (coal and coke) samples using loss on ignition/gain on ignition analysis. Such thermogravimetric analyzers include a furnace which heats the sample in a crucible to about 1000 degrees to evaporate the moisture and volatile constituents. The sample is weighed prior to being placed in the furnace and after it is heated to the desired temperature within the furnace. The weights are then compared to ascertain the amount of moisture and other volatile substances present in the sample.
The loss on ignition/gain on ignition analysis process has been automated by constructing the furnace with an opening on the top surface through which sample-containing crucibles can be inserted into and removed from the interior of the furnace. That allows the samples to be placed in the furnace and withdrawn from the furnace without opening the furnace door, thereby eliminating the heat loss and temperature fluctuation inherent in repeatedly opening the furnace door during the analysis.
Further, a carousel capable of retaining multiple sample-containing crucibles is provided within the furnace. The carousel supports the crucibles in the furnace while they are being heated in the furnace and then transports the crucibles, one at a time, into alignment with a scale for weighing.
An auto-loader can be provided for placing the crucibles on the internal carousel for heating and weighing, and for removing the crucibles from the internal carousel after they are weighed. An automated thermogravimetric analyzer of this type is disclosed in U.S. Pat. No. 7,172,729, issued Feb. 6, 2007 to applicant, which patent is incorporated herein by reference.
That patent discloses a robotic arm type auto-loader. However, other types of automatic loading mechanisms may be used to insert and remove the sample-containing crucibles from the furnace. For example, a more sophisticated auto-loader including a multi-crucible retaining external carousel, which is both rotatable and linearly movable toward and away from furnace opening, may be used. Such an auto-loader is provided in the Multi-matrix, multi-sample MMS-4000 TGA sold by Navas Instruments of Conway, S.C.
Thermogravimetric analyzers performing loss on ignition/gain on ignition analysis utilize ceramic crucibles with rounded interior bottom surfaces designed to cause the melted samples to accumulate at the lowest point on the bottom surface. However, such crucibles are not suitable for X-ray spectrometry analysis, as is explained below.
X-ray spectrometry analysis is also well known in the art for determining the composition of materials such as ores, cement and the like using X-ray fluorescence techniques. In order to perform X-ray spectrometry analysis, the sample is placed in a sample holder (crucible) suitable for X-ray spectrometry analysis and mixed with flux, such as lithium tetraborate, to form a homogeneous mixture. The crucible with the sample-flux mixture is heated and fused in a furnace to a high temperature. The homogeneous mixture is then normally poured to a casting dish for forming and cooling. The resulting material is called a fluxer.
The casting dish suitable for this process must prevent the bead from sticking to the sides of the dish after the fluxer bead has cooled. Once the bead has cooled, the casting dish with the fluxer bead is transferred to the X-ray spectrometry instrument for analysis. X-ray spectrometry requires that the casting dish have a flat bottom on which the cooled bead rests in order to provide an accurate result. Accordingly, the ceramic crucible with the rounded bottom normally used for the loss on ignition/gain on ignition analysis is not suitable for use in X-ray spectrometry analysis.
However, the loss on ignition/gain on ignition analysis and the X-ray spectrometry analysis are related because the accuracy of the X-ray spectrometry analysis is dependent upon having a sample with a known sample-to-flux ratio. Heating the sample to form the bead eliminates some of the volatile materials from the sample and therefore changes the sample-to-flux ratio of the sample.
It is therefore known to adjust the results of the X-ray spectrometry analysis to take into account the amount of volatile material in the sample. If the amount of volatile material in the sample is not known, it can be determined by first performing a loss on ignition/gain on ignition analysis on the sample in a thermogravimetric analyzer. The results of that analysis can then be provided to the computer associated with the X-ray spectrometry instrument. The computer will use the result of the loss on ignition/gain on ignition analysis to adjust for the loss of volatile materials from the sample, in order to increase accuracy of the X-ray spectrometry analysis.
Presently, when the loss on ignition/gain on ignition analysis is performed on a sample, the sample is split in two portions. One portion of the sample is used for the thermogravimetric analysis. The other portion of the sample is used for X-ray spectrometry analysis. The data from the loss on ignition/gain on ignition analysis of the first sample portion, performed in the thermogravimetric analyzer, is sent to the spectrometer for ratio correction.
The other portion of the sample is then weighed into a container and a measured amount of flux is added to the sample. The sample and flux are then agitated vigorously to obtain a homogeneous mixture and the homogeneous mixture, in the container, is placed in a furnace for fusing. After the sample is fused, the container with sample and flux is removed from the furnace the mixture is poured in the casting dish and allowed to cool to obtain the fluxer bead.
Ceramic crucibles suitable for use in the loss on ignition/gain on ignition analysis generally have a round bottoms. However, containers for use in X-ray analysis are made of metal and must have a non-stick surface and a flat bottom. Accordingly, preparation of samples for X-ray spectrometry analysis cannot be presently be performed in the thermogravimetric analyzer in which the loss on ignition/gain on ignition analysis is performed in a continuous, automated process because the crucibles used in the thermogravimetric analyzer are not suitable for use in X-ray spectrometric analysis.
That is so even if platinum crucibles were used for X-ray spectrometry. That is because the sample and flux must be thoroughly mixed obtain the homogeneous mixture necessary to form fluxed beads and the sample-flux mixing operation requires the removal of the sample-containing crucible from the furnace.
However, with the present invention, it is now possible to perform the loss on ignition/gain on ignition analysis and the X-ray spectrometry sample preparation in a single instrument, in a continuous, fully automated process. The present invention substantially reduces the amount of time, labor and expense of performing the loss on ignition/gain on ignition analysis and the X-ray spectrometry analysis sample preparation, both of which can now take place in a single instrument. In particular, the use of a unique sample holder and the use of a mechanism situated within the furnace to mix the sample and flux to obtain the homogenous mixture allows the entire process to be fully automated, such that both tasks can be performed on multiple samples in the furnace, without having to remove the samples for mixing, and thus without operator assistance.
It is, therefore, a prime object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometry analysis in a modified thermogravimetric analyzer.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometry analysis in a modified thermogravimetric analyzer capable of performing loss on ignition/gain on ignition analysis.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometry analysis in a modified thermogravimetric analyzer in which X-ray spectrometric analysis sample preparation can be performed with or without loss on ignition/gain on ignition analysis.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a modified thermogravimetric analyzer such that the loss on ignition/gain on ignition analysis value of the sample may be taken into account in performing X-ray spectrometry to compensate for the volatile material lost during heating of the sample.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a modified thermogravimetric analyzer wherein a single sample holder can be used for the loss on ignition/gain on ignition analysis and for the X-ray spectrometry analysis.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a modified thermogravimetric analyzer using a sample holder having a shape and made of material which prevents the cooled sample from sticking to it and which has a flat bottom suitable for X-ray spectrometry analysis thus eliminating the necessity for pouring molten samples from one container to another.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a modified thermogravimetric analyzer wherein a homogeneous mixture of the material to be analyzed and the flux is created by mixing the material to be analyzed and the flux within the furnace of the thermogravimetric analyzer.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a thermogravimetric analyzer which has been modified to repeatedly tilt the sample holders retained on the internal carousel in different directions as the carousel is rotated to transport sample holders into and out of alignment with the scale of the thermogravimetric analyzer to form the homogenous mixture necessary to obtain the bead.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a thermogravimetric analyzer which has been modified to agitate the contents of the sample holders by moving the sample holders back and forth rapidly with sudden stops after the sample holders have been tilted.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometry analysis in a modified thermogravimetric analyzer wherein multiple samples can be subjected to loss on ignition/gain on ignition analysis and preparation for X-ray spectrometric analysis in a single, continuous process.
It is another object of the present invention to provide a method and apparatus for preparing a sample for X-ray spectrometric analysis in a modified thermogravimetric analyzer wherein multiple samples can be automatically subjected to loss on ignition/gain on ignition analysis and preparation for X-ray spectrometric analysis without operator assistance.
It is another object of the present invention to provide a modified thermogravimetric analyzer suitable for sample preparation for X-ray spectrometry analysis.
It is another object of the present invention to provide a unique sample holder which can be used for both loss on ignition/gain on ignition analysis and for sample preparation for X-ray spectrometry analysis.